What Are Spur Gears?
Spur gears are the simplest and most widely used type of gear. They consist of a cylinder or disk with teeth projecting radially outward. The teeth are straight and parallel to the axis of rotation, making them easy to manufacture and highly efficient.
Key Terminology
Before designing spur gears, you need to understand the fundamental terms:
- Module (m): The ratio of the pitch diameter to the number of teeth. It defines the tooth size. Standard modules include 0.5, 1, 1.5, 2, 2.5, 3, 4, 5, 6, 8, and 10 mm.
- Pitch Diameter (d): The diameter of the theoretical circle where two gears mesh. Calculated as d = m × N (module × number of teeth).
- Pressure Angle (α): The angle between the tooth face and a tangent to the pitch circle. Standard values are 14.5°, 20° (most common), and 25°.
- Addendum: The height of the tooth above the pitch circle (equals 1 × module).
- Dedendum: The depth of the tooth below the pitch circle (equals 1.25 × module).
Involute Tooth Profile
Modern spur gears use an involute curve for the tooth profile. The involute is generated by unwinding a taut string from a circle (the base circle). This profile ensures constant angular velocity ratio between meshing gears and tolerates slight variations in center distance — making it ideal for practical applications.
Design Considerations
When designing spur gears, consider these factors:
- Minimum teeth: Gears with fewer than 17 teeth (at 20° pressure angle) may experience undercutting. Use profile shift correction for small tooth counts.
- Gear ratio: The speed ratio equals the number of teeth on the driven gear divided by the number on the driving gear.
- Center distance: For two meshing spur gears, center distance = (d1 + d2) / 2.
- Backlash: A small gap between meshing teeth to prevent jamming. Typically 0.03-0.05 × module.
Applications
Spur gears are used in clocks, washing machines, conveyor systems, power tools, rack-and-pinion mechanisms, and countless other applications where parallel shafts need to transmit motion.